1. Field of the Invention
The present invention relates to an optical apparatus and, more particularly, to an optical apparatus including a plurality of image pickup apparatuses using different forms of image pickup devices, an interchangeable lens applied to these image pickup apparatuses, and an aperture stop unit for the interchangeable lens.
2. Related Background Art
(1) Conventional Description of General Lens for Video Camera
A conventional most-well-known zoom lens for a video camera is made up of four lens units: a fixed positive lens unit, movable negative lens unit, fixed positive lens unit, and movable positive lens unit sequentially from the object side. In addition to this arrangement, zoom lenses with various lens arrangements are also known.
FIGS. 6A and 6B are sectional views showing the barrel structure of a zoom lens having the most popular four-lens-unit arrangement described above. FIG. 6B is a sectional view taken along the line 6B-6B in FIG. 6A.
The zoom lens is constituted by four lens units: a fixed front lens 201a, a variator lens unit 201b which moves along the optical axis to change the magnification, a fixed afocal lens 201c, and a focus lens unit 201d which moves along the optical axis to maintain the focal plane and adjusts the focus in changing the magnification.
Guide bars 203, 204a, and 204b are arranged parallel to an optical axis 205 to guide moving lenses and stop rotation of them. A DC motor 206 functions as a drive source for moving the variator lens unit 201b. FIGS. 6A and 6B show DC motors as a drive source for the variator lens unit 201b, but a stepping motor may be used similarly to a drive source (to be described later) for moving the focus lens unit 201d. 
The variator lens unit 201b is held by a holding frame 211. The holding frame 211 has a press spring 209, and a ball 210 engaged by the force of the press spring 209 in a screw groove formed in a screw bar 208. The screw bar 208 is rotated and driven by the DC motor 206 via an output shaft 206a and gear train 207, thereby moving the holding frame 211 in the optical axis direction along the guide bar 203.
The focus lens unit 201d is held by a holding frame 214. A screw member 213 is integrated into the holding frame along the optical axis near the sleeve (element fitted in the guide bar to form a guide) of the holding frame 214. A stepping motor 212 rotates an output shaft 212a of the stepping motor 212. An external thread formed on the output shaft 212a, and an internal thread or rack formed in the screw member 213 interlock with this rotation. By this interlocking rotation, the screw member 213 allows the holding frame 214 to move in the optical axis direction along the guide bars 204a and 204b. A detailed structure of the coupling portion between the holding frame 214 and the screw member 213 is disclosed in Japanese Patent Application Laid-Open No. 4-136806.
As described above, the interlocking mechanism by the stepping motor 212 may be a variator driving mechanism.
One reference position along the optical axis of the holding frame may be set detectable by a photointerrupter (not shown) and a light-shielding wall integrated with the holding frame, in order to detect the absolute position of a moving lens along the optical direction when moving the lens by using this stepping motor 212. In this case, a position detection means can be constituted which detects the absolute position of the holding frame by successively counting the number of drive steps applied to the stepping motor 212 after the holding frame is located at the reference position.
In addition to the DC motor 206 and stepping motor 212, some known arrangements adopt a linear actuator of moving coil (or magnet) type that is made up of a coil (or magnet) attached to the holding frame and a magnet (or coil) attached to the fixed side.
(2) Conventional Description of Image Pickup Apparatus
FIG. 7 is a block diagram showing the electrical arrangement of a conventional image pickup apparatus. The same reference numerals as in FIGS. 6A and 6B denote parts having the same functions.
In FIG. 7, the image pickup apparatus comprises a solid-state image pickup device 221 such as a CCD. A zoom drive source 222 for the variator lens unit 201b includes the DC motor 206, the gear train interlocked with the DC motor 206, and the screw bar 208 in FIG. 6A. Alternatively, the zoom drive source 222 is comprised of a stepping motor similarly to the drive source of the focus lens unit 201d in FIGS. 6A and 6B. A drive source 223 for the focus lens unit 201d includes the stepping motor 212, the output shaft with an external thread, and the screw member 213 integrated with the holding frame along the optical axis.
The image pickup apparatus further comprises an aperture stop drive source 224. A zoom encoder 225 and focus encoder 227 detect the absolute positions of the variator lens unit 201b and focus lens unit 201d along the optical axis. When the variator drive source is realized by a DC motor, as shown in FIGS. 6A and 6B, an absolute position encoder such as a volume (not shown in FIGS. 6A and 6B) may be used. This encoder may be of magnetic type. When a stepping motor is used for the drive source, it is general to locate the holding frame at a reference position and successively count the number of operation pulses input to the stepping motor, as described above.
An aperture stop encoder 226 is, e.g., one which incorporates a Hall element in a motor serving as an aperture stop source and detects the relationship in rotational position between the rotor and the stator. A camera signal process circuit 228 performs predetermined amplification and gamma correction for an output from the CCD 221. A contrast signal of an image signal having undergone the predetermined process passes through an AE gate 229 and AF gate 230. That is, these gates set from the entire frame a signal extraction range optimal for determining the exposure and adjusting the focus. The gate size is variable and in some cases a plurality of gates are adopted, but a detailed description thereof will be omitted for descriptive convenience.
An AF signal process circuit 231 for AF (Auto Focus) generates one or a plurality of outputs concerning the high-frequency component of an image signal. The image pickup apparatus has a zoom switch 233. A zoom trucking memory 234 stores information about a prospective focus lens position corresponding to the object distance and variator lens position in changing the magnification. The zoom trucking memory may be a memory in the CPU. A CPU 232 controls various circuits.
In this arrangement, when the user operates the zoom switch 233, the CPU 232 drives and controls the zoom drive source 222 and focus drive source 223 such that the current absolute position of the variator lens unit 201b along the optical axis as the detection result of the zoom encoder 225 coincides with a calculated prospective position of the variator lenses, and the current absolute position of the focus lens unit 201d along the optical axis as the detection result of the focus encoder 227 coincides with a calculated prospective position of the focus lenses, so as to maintain a predetermined positional relationship between the variator lens unit 201b and the focus lens unit 201d that is calculated based on information in the zoom trucking memory 234.
In autofocus operation, the CPU 232 drives and controls the focus drive source 223 such that an output from the AF signal process circuit 231 exhibits a peak.
To obtain correct exposure, the CPU 232 drives and controls the aperture stop source 224 and controls the aperture diameter based on an output from aperture stop encoder 226 so as to set the average value of outputs of Y signals (brightness signals) having passed through the AE gate 229 to a predetermined value.
(3) Conventional Description of Television Signal Autofocus Method in Image Pickup Apparatus with Above Arrangement
This method has no drawback such as necessary for without any cost for another sensor because an autofocus sensor also serves as the image pickup device of an image pickup apparatus. In this method, since an image state on an imaging plane is directly detected, for example, even when a temperature change expands or contracts the lens barrel to change the focal position, a correct focus position can be detected in accordance with this change.
FIG. 8 shows this principle. The abscissa represents the lens position for focus adjustment, and the ordinate represents the high-frequency component (focal point voltage) of an image pickup signal. The focal point voltage exhibits a peak at a position indicated by the arrow in FIG. 8. This position A is an in-focus lens position.
An example of obtaining the focal point voltage F will be described.
FIG. 9A shows an actual image pickup field having a view angle 320, a range 318 for extracting an image pickup signal for autofocus adjustment, and an object 319 to be sensed.
In FIG. 9B, (a) represents the state of the object in the image pickup signal extraction range, and (b) represents an image pickup (brightness) signal (Y signal) of the object in (a). This signal is differentiated into a waveform (c), and absolutized into a waveform (d). A sampled and held signal (e) of the resultant signal is the focal point voltage F. This utilizes the fact that particularly the high-frequency component of the contrast signal of an object to be sensed maximizes in an in-focus state. The focal point voltage generation method includes various methods in addition to this method.
A high-pass filter is often used to extract only a high-frequency component. It is also known that a filter having several kinds of characteristics is prepared, focal point voltages are set for a plurality of frequencies, and a correct focus is secured based on these pieces of information.
FIG. 10 is a view showing the main part of an image pickup apparatus as a combination of such an autofocus adjustment apparatus and an inner focus lens.
An image pickup device such as a CCD is located at an imaging position 505. A brightness signal Y is generated by a signal process circuit (not shown) or the like on the basis of an output from the image pickup device, and information within a predetermined frame is received by an AF circuit 521. The AF circuit 521 obtains a focal point voltage by the above-described method or the like. An in-focus or out-of-focus state, and a rear- or front-focus state for the out-of-focus state are determined based on the focal point voltage value, the driving direction of a focus lens 504B, and the sign of a change in focal point voltage upon drive. A focus lens drive motor 522 is driven in a predetermined direction on the basis of the determination result. In FIG. 10, a front lens 501, variator lens 502, and afocal lens 504A are arranged.
This autofocus method is called a “television signal autofocus” in which the image sensor of the image pickup apparatus also serves as an autofocus sensor. Thus, the imaging state of the imaging plane is directly measured, and the focal state can always be grasped with high precision. To determine a far- or near-focus state by this method when the focus greatly deviates, the focus lens is vibrated by a predetermined small amount along the optical axis to measure an increase or decrease in focal point voltage signal. Even if the signal increase or decrease cannot be obtained, the focus lens is driven in either direction to measure a signal change. In this method, a relatively long time is taken from an out-of-focus state to an in-focus state.
(4) Conventional Description of Zoom Trucking
As shortly described in (2), the focus lens during zoom takes a different trucking locus in accordance with the object distance in focusing by lenses behind a variator lens in an image pickup apparatus having the arrangement as shown in FIG. 7. For this reason, the focus must be maintained even during zoom by measuring the absolute positions of the variator and focus lenses along the optical axis at the start of zoom, specifying a prospective positional relationship between the two lenses during zoom, and performing operation so as to keep these positions. This operation is called zoom trucking.
As this method, Japanese Patent Application Laid-Open No. 1-321416 discloses the following method. More specifically, focus lens positions for a plurality of variator lens positions between the wide angle end and the telephoto end are stored in accordance with a plurality of object distances. At the start of zoom, the current variator and focus lens positions are checked on map information stored in a memory means in a microcomputer. Interpolation calculation is executed based on data stored nearest to the front-focus side from the obtained point with the same focal length, and data stored nearest to the rear-focus side. Focus lens positions are calculated for respective focal lengths (variator lens positions).
FIG. 11 is a view of the trucking curve (locus) near the telephoto end. The abscissa represents the variator lens position; Vn, the telephoto end position; and the ordinate, the focus lens position.
For example, P1, P4, P7, and P10 are stored for an infinite distance, and P2, P5, P8, and P11 are stored for 10 m. If the lens is moved for zoom toward the wide angle end while the focus lens position is at point P (object distance is between 10 m and the infinity at the telephoto end), the positional relationship between the variator lens and the focus lens is controlled to sequentially track PA, PB, and PC from P. The positions PA to PC are positions where the interpolation ratio between upper and lower stored loci LL1 and LL2 is constant.
(5) Conventional Description of Lens-Interchangeable Image Pickup System
It is well known that a lens can be interchanged from an image pickup apparatus body in an image pickup apparatus having the above arrangement.
FIG. 12 is a block diagram showing an example of a lens-interchangeable image pickup system. A zoom lens made up of four, positive, negative, positive, and positive lens units from the object side will be exemplified, similar to the above description, but the image pickup system may employ another arrangement.
In FIG. 12, the lens side comprises a fixed front lens 111, a variator lens 112 which moves along the optical axis to change the magnification, an aperture stop unit 136, and a fixed afocal lens 113. A focus lens 114 performs focus operation when the object distance changes, and also functions as a compensator during zoom. Drive sources 145, 413, and 137 are for the variator lens 112, aperture stop unit 136, and focus lens 114, respectively, and are driven and controlled by a lens microcomputer 410 via drive circuits 161, 414, and 162, respectively.
In this example, the image pickup apparatus body side comprises three image pickup devices 303 to 305 such as CCDs. Their image pickup signals are amplified by amplifiers 405 to 407, and processed into an image pickup signal by an signal process circuit 152. The image pickup signal is transmitted to an apparatus microcomputer 409.
The two microcomputers 409 and 410 are connected via a transmission path by contact between contacts 318 and 307. The microcomputers 409 and 410 exchange various signals through this transmission path.
If the signal process circuit 152 generates a focal point voltage for the above-mentioned television signal autofocus in this arrangement, the information is transmitted from the apparatus microcomputer 409 to the lens microcomputer 410. The lens microcomputer 410 determines an in-focus or out-of-focus state on the basis of the signal information, or determines the direction and drive speed of the focus lens 114 depending on the direction (rear- or front-focus state) and degree of the out-of-focus state. Then, the lens microcomputer 410 drives the drive source 137 via the drive circuit 162.
(6) Conventional Description of Image Pickup Device
An image pickup device such as a CCD is attaining a diagonal length of about 6 mm or 4 mm that is called ⅓″ or ¼″ in a consumer video camera. The image pickup device has 310,000 pixels within this size. A digital still camera uses a so-called megapixel CCD of 2,000,000 pixels within about ½″ (diagonal length: about 8 mm), and is ensuring an image quality almost equal to that obtained by a conventional silver halide camera for at least a popular small print size.
The diameter of permissible circle of confusion is about 12 to 15 μm in the video camera and about 7 to 8 μm in the digital still camera. This is a very small numerical value in comparison with a diameter of permissible circle of confusion of 33 to 35 μm for a 135 film format because the diagonal size of the frame is much smaller than a 43-mm diagonal size of a silver halide camera. This value is predicted to decrease more in the future.
From another viewpoint, the focal length for obtaining the same angle of view can be shortened in an image pickup apparatus using such a CCD in comparison with a 135 film camera because of a small image size. The angle of view obtained at a standard focal length of 40 mm by the 135 film is 4 mm in a camera using a ¼″ CCD. Thus, the depth of field in image pickup with the same F value is very long in the image pickup apparatus using the CCD.
As is well known, the depth of field is determined by “permissible circle of confusion×F”. For example, for F=2, the depth of field is “0.035×2=0.07 mm” in a 135 film camera, but “0.07×2=0.14 mm” in a ½″ digital still camera which is ⅕ that of the 135 film camera.
As for an image pickup device such as a ⅓″ CCD having the same diagonal line, e.g., 6 mm, there are known various forms of image pickup devices such as CCDs including image pickup devices which attain higher resolution by increasing the number of pixels to one million pixels or in the future two to several million pixels, and image pickup devices which ensure the pixel size, sensitivity, and dynamic range while slightly suppressing the number of pixels.
In this manner, a CCD to be used changes depending on whether high resolution is important or the sensitivity or dynamic range is important. In addition, the form is also changed depending on whether the optical path must be shielded by a mechanical shutter during charge transfer (read of all pixels or not).
For example, a film camera selectively uses a low-sensitivity, ultra-fine-grain film which puts emphasis on high-resolution image pickup, and a high-sensitivity film with lower resolution in accordance with the intended use. Similarly, a CCD to be used also changes depending on the image pickup purpose.
As is well known, the optical diffraction phenomenon occurs when light passes through a small hole. In the image pickup apparatus, the image quality is readily decreased by the diffraction phenomenon with the use of a megapixel CCD of a small pixel pitch. For example, even with the same ⅓″ CCD, a video camera of a large pixel pitch (small number of pixels) can be practically used up to F16 or F22, but a digital camera of 1,000,000 pixels can only be used up to F8 or F11, and a digital camera of 2,000,000 pixels can only be used up to F5.6 or F8. Although these numerical values are merely an example, the F value which causes diffraction of a small aperture changes to a smaller F value as the resolution qualitatively increases.
To prevent this, the F value as a limit aperture diameter is set to a unique value in a lens-integrated image pickup apparatus such as a video camera so as not to use a smaller F value.
However, when the same lens interchangeable from an image pickup apparatus can be mounted on different image pickup apparatuses with image pickup devices, such as a video camera, a digital still camera with a higher-resolution CCD, and a digital still camera with higher sensitivity, or when the image pickup device can be interchanged from the image pickup apparatus in accordance with the intended use, the following problems arise unless the F value as a limit aperture diameter is set changeable in accordance with the CCD form (e.g., the numerical value of the pixel pitch). a) The image degrades due to the diffraction phenomenon depending on a selected CCD. b) The exposure adjustable range by the aperture stop unit is excessively limited depending on a selected CCD.